Aside from the Google places method, one way we have great success with is using smaller business directories like MerchantCircle, Manta, and etc.
Pick a niche you want to target and develop a document showing them "exactly" how to optimize their listing to the fullest. Almost none of them have completed listings. Be sure and show the wrong way, the missing information, & etc right next to the right way and why filling everything out helps. Also, the "perfect" listing you compare the "bad" one to would be your listing. Call it free advertising, with out advertising
You can then just email clients with the header "Your ABC directory listing" and give the report away for free (Put a small sales pitch below your signature). We get many calls thanking us for the report, and asking us to do it for them ... charge cheap because it is the in you need to make them happy and upsell your true service.
If your a cold caller, call as a "Verification Listing Specialist". Go over their information, and you just "happen" to notice they are missing things. Ask if they would like to receive a report showing them how to get the most out of their listing. Almost all say yes. Send it, and let them know they can contact you if they need any help. Many will call back. Others you can follow up with to see if they have any questions on enhancing their listing, and you can get more to help. Again, this is a in you can charge $29-$99+, as you "in" service ... wow them, then upsell.
Our telemarketers are doing very well with this approach because you are not "selling" anything. You are just verifying listings, and offering free help.